The invention relates to a method of extracting natural materials, especially hops.
It has long been known that substances contained in hops can be recovered by extracting the hops with a suitable extractant, such as organic solvents or, in recent times, carbon dioxide, in the subcritical and/or supercritical state. On account of the very low bulk weight of the ground raw hops, of 0.15 kg/l, and for the purpose of reducing the high investment cost involved in the extraction pressure vessel and keeping the transport and storage costs low, it is known to compress the raw hops which are to be extracted with carbon dioxide. This is accomplished first by grinding the raw hops and then pressing to form pellets of a diameter of about 6 mm and a length of about 20 mm. By these means a three- to four-fold compression can be attained. These commercially common hop pellets of the stated dimensions, however, cannot be used directly for extraction with carbon dioxide, since the yield, especially of the desired bitter substances, is too low. In the "Monatsschrift fur Brauerei" (180), page 107, it is stated that, for this reason, the hop pellets are first ground, in order then to be able to extract the powder with good yields. However, on account of the grinding machines that are additionally needed, this process calls for greater technical investment. Furthermore, it suffers from the considerable disadvantage that the hops are reheated by the grinding of the pellets, resulting in losses of the desired alpha acids, and also that charging the extraction autoclave with powder fosters tunneling in the extraction bed. The result of this is that, on the one hand, the carbon dioxide flows through these tunnels without becoming laden with the extractable substance, while on the other hand the carbon dioxide does not reach the more tightly packed areas of the bed, resulting in severe losses of the yield of hop extract. Similar problems may also occur in the extraction of substances from other natural materials.
To eliminate the tunneling that occurs in the extraction bed, attempts have already been made to provide the extraction autoclaves with stirrers in order to assure a uniform distribution of the powder. This, however, requires a heavy investment in apparatus in order to achieve a good seal around the stirrer for operation at pressures of as much as 500 bar.
Accordingly, only two alternatives have heretofore been available, namely either to use the ground raw hops whose bulk weight is as unfavorable as in the case of the raw hops themselves, or to use ground pellets for the extraction (J. Instr. Brew. Vol. 87 (1981) 24 to 29, especially 27, column 1, 3rd paragraph). These two alternatives, however, suffer from the disadvantages, described above, of incomplete extraction due to tunneling.
The object of the present invention, therefore, is to devise a method for the extraction of natural materials, especially hops, which will make possible a very complete recovery of the desired substances by extraction with carbon dioxide in a simple manner, without impairment of the valuable extracts.